The so-called “lost-foam” casting process is a well-known technique for producing aluminum castings, wherein a fugitive, pyrolizable, polymeric foam pattern [e.g. expanded polystyrene foam (EPS)] is covered with a thin (i.e. 0.25-0.5 mm), gas-permeable, refractory (e.g. mica, silica, alumina, alumina-silicate, etc.) coating, and embedded in compacted, unbonded sand to form a pattern-filled, mold cavity within the sand. The most popular pattern foam for casting aluminum is high mass average molecular weight (i.e. Mw>200,000) expanded polystyrene (EPS). Molten aluminum (hereafter “melt”) is introduced into the pattern-filled mold cavity to liquefy, thermally degrade and displace the pattern with melt. Gaseous and liquid decomposition/pyrolysis products from the thermally degraded foam escape through the gas-permeable refractory coating into the interstices between the unbonded sand particles.
The melt may be either gravity-cast (i.e. poured from an overhead ladle or furnace), or countergravity-cast (i.e. forced upwardly by vacuum or low pressure into the mold cavity from an underlying vessel (e.g. a furnace). Faster casting rates (i.e. the rate at which the melt enters the mold cavity) result in less heat loss during pouring. Less heat loss during pouring keeps the melt hotter, which in turn, reduces the formation of casting defects such as “folds” (i.e. thermal degradation products trapped at the confluence of cold metal fronts), “cold shuts” (i.e. sites where metal does not completely fill the pattern due to premature solidification), and “gas inclusions” (i.e. pyrolysis gases entrapped in solidified melt before they can escape the mold cavity).
The EPS foam pattern is made by injecting pre-expanded polystyrene beads into a steam-heated pattern mold to impart the desired shape to the pattern. Complex patterns are made by separately molding several individual/discrete mold segments (a.k.a. “slices”), and then gluing the individual segments together to form a finished “pattern assembly”. Paraffinic hot melt glues are commonly used to form pattern assemblies, but can contribute to the formation of casting defects. In this regard, the paraffinic glues tend to resist thermal degradation ahead of the melt front advancing into the pattern. Undegraded glue in the mold cavity impedes advancement of the melt front which, in turn, promotes localized heat loss, and the associated formation of the aforesaid casting defects, as well as defects resulting from entrapment of any solid glue residues in the casting.